(a) Technical Field
The present invention relates to a heating device for an end plate of a fuel cell stack. More particularly, it relates to a heating device for an end plate of a fuel cell stack, which can prevent a decrease in temperature of unit cells around the ends of the fuel cell stack by providing a structure for circulating high temperature coolant discharged from the fuel cell stack in the end plate. A non-uniform temperature distribution in the fuel cell stack can thereby be prevented.
(b) Background Art
First, the configuration of a fuel cell stack (hereinafter referred to also as stack) will be briefly described with reference to FIGS. 6 and 8 below.
A membrane-electrode assembly (MEA) is positioned in the center of each unit cell of the fuel cell stack, and the MEA comprises a solid polymer electrolyte membrane 10, through which hydrogen ions (protons) are transported, and an electrode/catalyst layer such as a cathode (“air electrode”) 12 and an anode (“fuel electrode) 14, in which an electrochemical reaction between hydrogen and oxygen takes place, disposed on both sides of the polymer electrolyte membrane 10.
Moreover, a gas diffusion layer (GDL) 16 and a gasket 18 are sequentially stacked on both sides of the MEA, where the cathode 12 and the anode 14 are located. A separator 20 including flow fields, through which reactant gases (such as hydrogen as a fuel and oxygen or air as an oxidant) are supplied and coolant passes, is located on the outsides of each GDL 16.
After several hundreds of unit cells are stacked, an end plate 30 for supporting and fixing the unit cells is connected to each end of the fuel cell stack.
Further, a current collector 32 for collecting electricity generated in the stack and supplying the electricity to the outside is mounted on the inside of each end plate 30.
An oxidation reaction of hydrogen occurs at the anode 14 of the stack to produce hydrogen ions (protons, H+) and electrons (e−) by a catalyst disposed in the electrode/catalyst layer. The hydrogen ions and electrons are transmitted to the cathode 12 through the electrolyte membrane 10 and the separator 20. At the cathode 12, water is produced by the electrochemical reaction between the hydrogen ions and electrons transmitted from the anode 14 and the oxygen-containing air. Electrical energy generated by the flow of electrons is supplied to a load that uses the electrical energy through the current collector 32 of the end plate 30.
Hydrogen inlet and outlet manifolds, air inlet and outlet manifolds, and coolant inlet and outlet manifolds are further formed adjacent to each other on the separators 20 as well as the end plates 30.
The flow of coolant for cooling the unit cells of the stack is as follows. As shown in FIG. 6, the coolant supplied through a coolant inlet manifold 34 cools the unit cells of the stack and is then discharged through a coolant outlet manifold 36.
However, when measuring the temperature of the coolant in the coolant outlet manifold 36, it can be seen from the graph of FIG. 7, which displays the temperature for the various cells in the stack, that the temperature of the coolant at the upstream is high and gradually decreases at the downstream. In other words, the temperature of the coolant increases as it cools the unit cells of the stack, and then gradually decreases as the coolant flows toward the outlet of the end plate through the coolant outlet manifold.
If the temperature of the coolant decreases as the coolant flows toward the outlet of the end plate through the coolant outlet manifold, the temperature of unit cells adjacent to the end plate also decreases, and thus a non-uniform temperature distribution occurs in the entire unit cells.
A typical polymer electrolyte membrane fuel cell generally exhibits excellent performance in a temperature range from room temperature to 80° C. However, if the non-uniform temperature distribution occurs in the entire unit cells as the temperature of several unit cells is lowered, the performance is reduced by a reduction in reaction activity and a reduction in ion conductivity of the electrolyte membrane.
In particular, if the temperature of the stack mounted in a vehicle is lowered below the freezing point (e.g. if the outside temperature is below zero such as in winter conditions), the activity of the electrodes including the cathode and the anode in the stack is reduced. Moreover, the water carrying hydrogen ions in the electrolyte membrane freezes in the stack, which reduces the ion conductivity of the electrolyte membrane, thereby deteriorating the performance of the stack.
Further, if the temperature of the stack is low while humidified gas is supplied to the stack, a flooding problem occurs due to condensation of water. This has a critical effect on the performance and durability of the stack. Therefore, in order to operate the fuel cell stack at an appropriate temperature, it is very important to uniformly maintain the temperature distribution of the fuel cell stack, in which several hundreds of unit cells are stacked together, in a predetermined range.
Taking these factors into account, many methods have been devices in an attempt to prevent a decrease in temperature of unit cells adjacent to the end plate. For example, methods have been proposed wherein a device is inserted for thermally insulating or heating the area between the end plate and the stacked cells.
For example, U.S. Pat. No. 6,824,901 describes a method of inserting a thick insulator between an end plate and a separator to thermally insulate the region where the reaction occurs, or disposing a plane heater between the end plate and the separator to maintain the temperature of the entire fuel cell stack at a predetermined level during cold start-up.
Korean Patent No. 10-2006-0077284 describes a fuel cell stack, in which different types of current collectors having different coefficients of thermal expansion are provided to generate heat, thereby thermally insulating unit cells around the end plate.
Korean Patent No. 10-2006-0074397 describes a stack fixture structure for cold start-up of a fuel cell vehicle, in which a cover for covering the outside of an end plate is attached to a fuel cell stack to form an air layer for thermal insulation.
However, in the case where the entire end plate is thermally insulated, the thickness of the insulator for the thermal insulation should be increased, which increases the thickness of the entire fuel cell stack. In the case where the cover is attached to the outside of the end plate, it is impossible to prevent the heat generated in the electrodes from being transferred to the end plate. Moreover, in the case where the heater is disposed between the end plate and the separator, it is necessary to supply an external power source for the operation of the heater, and thus a system for controlling the heater and power supply is complicated.
Accordingly, there remains a need in the art for an apparatus and method for maintaining a uniform temperature distribution in a fuel cell stack.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.